This grant seeks to increase our understanding of Hepatitis C virus (HCV) sexual transmission by studying five HCV sexual transmission pairs. Data suggests that HCV can replicate in several anatomic compartments including the liver, white blood cells, and within the genital tract (1-3). Furthermore, viruses replicating in these different compartments can be characterized by small differences in their sequences. The primary aim of the research is to determine where in the transmitting partner the HCV that established infection in the recipient came from by determining their sequence characteristics. Following identification of transmission pairs where HCV was likely acquired via sex, samples of bodily fluids that may be involved in transmission will be collected from the source partner including semen and blood. Blood will also be collected from the subject in the pair who acquired HCV infection. Following isolation of HCV from these fluids we will use massively parallel sequencing methods to generate a detailed sequence signature for the HCV in the various compartments. We will then compare these sequence signatures between the transmitting and recipient partners to determine where the virus that infected the partner likely originated in the transmitter. A second aim of the research is to determine whether difference exist in HCV sequences and distribution between persons who are co-infected with HIV and those who have HCV alone. To accomplish this an additional 5 subjects will be enrolled so that we can analyze 5 HCV and 5 HCV/HIV chronically infected subjects who either transmitted HCV or would be possible HCV transmitters.